What Determines an Object's Transparency and Reflectivity?

[Rainbow]

What is the factor that determines whether an object will be transparent, translucent, or opaque?
Also, what makes reflecting surfaces reflect EM rays?

 

[Danger]

Both factors are based upon how the atomic structure of the material interacts with photons of various energy levels (frequencies). Photons are absorbed by atoms, then new ones radiated in some specific direction. Reflection entails the re-radiated particles coming out the same side of the atom that the originals entered. The real scientists around here can provide clarification and even formulae, but it's a start.